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Raman spectroscopy and single-crystal X-ray diffraction studies of sodium dithionite in the solid state reveal that sodium dithionite exists in different forms. In one anhydrous form, the dithionite ion has C
2 geometry, almost eclipsed with a 16° O-S-S-O torsional angle. In the dihydrated form (Na
2S
2O
4·2H
2O), the dithionite anion has a shorter S-S bond length and a gauche 56° O-S-S-O torsional angle.[2]

The weak S-S bond causes the dithionite anion to dissociate into the [SO2]− radical anion in aqueous solution, which has been confirmed by ESR spectroscopy. It is also observed that 35S undergoes rapid exchange between S2O42− and SO2 in neutral or acidic solution, consistent with the weak S-S bond in the anion.[3]

Sodium dithionite is stable when dry, but is slowly oxidized by air when in solution. Even with the absence of air, solutions of sodium dithionite deteriorate due to the following reaction:

2 S2O42− + H2O → S2O32− + 2 HSO3−

Thus solutions of sodium dithionite cannot be stored for a long period of time.[3]

Anhydrous is a monoclinic crystal with slightly sulfuric odor. It is soluble in water and slightly soluble in ethanol. Dihydrate is a columnar crystal, and it is so unstable that it easily dehydrates to anhydrous and is easily oxidized by oxygen in the air.

Powdered anhydrous sodium dithionite with a small amount of water may ignite in air by the heat of decomposition. In absence of air (oxygen), it only decomposes slowly.

An aqueous solution of sodium dithionite is acidic and decomposes to sodium thiosulfate and sodium bisulfite. The reaction rate increases with increasing temperature. In addition, the rate is higher under stronger acidity.

Sodium bisulfate and sodium bisulfite decrease the pH and therefore accelerate the reaction. Sulfur dioxide is formed under strongly acidic conditions.

2 H2S2O4 → 3 SO2 + S + 2 H2O

3 H2S2O4 → 5 SO2 + H2S + 2 H2O

On the other hand, alkali solution (pH 9–11) is stable and only decomposes about 1% per an hour. It has strong reducing properties and decomposes to sulfurous acid and sulfide.

3 Na2S2O4 + 6 NaOH → 5 Na2SO3 + Na2S + 3 H2O

In the presence of aldehydes, sodium diothionite reacts either to form α-hydroxy-sulfinates at room temperature or to reduce the aldehyde to the corresponding alcohol above a temperature of 85 °C.[4][5] Some ketones are also reduced under similar conditions.

This compound is a water-soluble salt, and can be used as a reducing agent in aqueous solutions. It is used as such in some industrial dyeing processes, primarily those involving sulfur dyes and vat dyes, where an otherwise water-insoluble dye can be reduced into a water-soluble alkali metal salt (e.g. indigo dye).[6] The reduction properties of sodium dithionite also eliminate excess dye, residual oxide, and unintended pigments, thereby improving overall colour quality.

Sodium dithionite can also be used for water treatment, gas purification, cleaning, and stripping. It can also be used in industrial processes as a sulfonating agent or a sodium ion source. In addition to the textile industry, this compound is used in industries concerned with leather, foods, polymers, photography, and many others. Its wide use is attributable to its low toxicity LD50 at 5 g/kg, and hence its wide range of applications. It is also used as decolourising agent in organic reactions.

Sodium dithionite is often used in physiology experiments as a means of lowering solutions' redox potential (Eo' -0.66 V vs SHE at pH 7).[7]Potassium ferricyanide is usually used as an oxidizing chemical in such experiments (Eo' ~ .436 V at pH 7). In addition, sodium dithionite is often used in soil chemistry experiments to determine the amount of iron that is not incorporated in primary silicate minerals. Hence, iron extracted by sodium dithionite is also referred to as "free iron." The strong affinity of the dithionite ion for bi- and trivalent metal cations (M2+, M3+) allows it to enhance the solubility of iron, and therefore dithionite is a useful chelating agent.

Sodium dithionite has been used in chemical enhanced oil recovery[citation needed] to stabilize polyacrylamide polymers against radical degradation in the presence of iron. It has also been used in environmental applications to propagate a low Eh front in the subsurface in order to reduce components such as chromium.